Windows may incorporate a transparent solar collector and thereby offer a more pleasing architectural aesthetic than opaque solar heat collectors. These windows typically include two glass sheets between which either air or a liquid is received—the latter being described in GB2450474. A problem experienced with these liquid-circulating transparent collectors has been the necessity for connecting the window to a remote heat exchanger in which the energy-absorbing fluid piped from the collector transfers heat energy to some other fluid for utilisation. This has involved an undesirably large amount of plumbing, pump and pipe fittings, together with the additional volume of heat absorbing fluid required to fill such plumbing, as well as the labour and materials in the piping and assembly thereof. It also has involved excessive opportunity for leakage, property damage and loss of comparatively expensive energy-absorbing working fluid, say when a transparent pane is broken. It is an object of the present invention to overcome or substantially ameliorate the above disadvantages or more generally to provide an improved heat absorbing window.